Thin film deposition techniques, such as plasma enhanced chemical vapor deposition (PECVD), and apparatus therefore have been around for a number of years. They are useful in the production of many products.
One such use is in deposition of hard coatings. Since its inception, there has been a demand for hard thin film coatings for use in the cutting tool, ball bearing and military armor industries. In the high tech world in which we live today, the demands for these type of films are even greater with applications covering a vast number of unconventional fields. For example, there is a need in the computer field for protective coatings for magnetic tape heads and hard CD disks. Also the ever growing telecommunications field requires hard films to protect optical fibers. Recently, there has been a large demand from the medical industry for use of hard thin coatings for such applications as orthopedic devices and wear resistant teeth. With the large number of applications for these types of films, improvements in the wear properties of these thin film coatings can drastically increase the lifetimes of many of the above mentioned products saving billions of dollars. Thus while there have been many advances in the hard thin film coating field over the past few decades, there is always a demand for further improvement of the tribological properties of these materials.
The use of low energy bombardment during the growth of these types of films has been found to have a strong affect on the wear properties of resulting film. In particular, the microhardness of diamond-like coating has been found to substantially increase with an increase in flux density of the ions bombarding the film surface during growth. Similar improvements in the tribological properties of boron nitride, titanium nitride, tungsten and copper with increased ion bombardment have been reported. However, to obtain these high flux conditions using standard chemical vapor deposition processes, one must use high applied powers and high chamber pressures which lead to undesirable film properties. In the case of diamond-like carbon films, gas phase polymerization occurs under these conditions leading to the formation of graphitic-like structures in the films leading to poor tribological properties. For boron nitride films, detrimental hydrogenated and amorphous phases appear in the films under these conditions. In addition, the ion energies in these processes under the high power conditions are unacceptably high, leading to sputtering, point defect or defect cluster formation and other undesirable effects. As an alternative, conventional ion beam sources have been used in conjunction with film deposition processes to produce the desired ion fluxes and bombarding conditions. However, the surface area coverage of these types of ion sources are limited and the sources themselves can be somewhat costly.
Because there is such a broad base of interest in wear resistant thin films, the benefits of improved tribological properties for these films or advances in the process used to prepare them with would be far reaching. Improvements in the computer, telecommunications and medical products listed previously are obvious. Mass-produced goods, such as micro-machined sensors and actuators, cutting and machining tools and even ordinary ball bearings, whose performance depends upon the characteristics of a protective thin-film coating, could be improved. In addition, for films that can be made to be transparent, the improved coating could be applied to increase the protection of already-hard glass or ceramics used for demanding applications such as airplane windshields and canopies. A new process producing superior quality films could also affect fields besides those interested in only wear resistance. For example, the making of diamond films, which are used as part of heat sinks for semiconductor devices and PC boards because of their high thermal conductivities and strong insulating properties, could benefit from a new technique as well.
Thus, the economic impact to a country whose industry depends on leading-edge production technologies is enormous. A specific agency in the federal government, which could strongly benefit from this improved technology, is the Department of Defense, through the acquisition of otherwise fragile sensors that must survive the rigors of the field.
Another useful area for thin film deposition is photovoltaics. Photovoltaics (PV), the direct conversion of sunlight into electricity, has long been a principal source of electrical energy for use in space applications. It's practical use on earth has traditionally been limited to only small area applications with large scale use being too costly. However, a growing awareness of and concern over the environmental consequences of conventional fossil-fuel and nuclear power and the desire to reduce reliance on imported oil have increased the need for practical, affordable renewable energy. In PV research and development, much of the focus has been on thin film solar cell technology because of the potential of preparing large area modules at low temperatures at reduced material costs over the standard crystalline silicon modules. Amorphous Silicon (a-Si:H) technology stands out in the thin film field due to the relatively simple, inexpensive deposition process used to produce solar modules, the environmentally safe materials in the final product and the ability to use light-weight, flexible substrates to create the modules.
While many advancements in a-Si:H PV technology have been made by Energy Conversion Devices, Inc. of Troy Michigan (ECD) and other research groups, the highest present day stable efficiencies for small area (0.25 cm.sup.2) triple-junction cells are around 13% and 10% efficiencies have been obtained for 4 ft.sup.2 modules. In order to improve these efficiencies and the performance of other a-Si:H based devices (thin film transistors, photosensors, etc.), several issues related to the a-Si:H material and its alloys must be addressed:
1) Low Carrier Mobilities and Poorer Carrier Collection
The carrier mobilities of a-Si:H are rather low compared with those for crystalline materials. For device quality material, the electron mobilities are between 0.5 to 1.5 cm.sup.2 Vs, more than two orders of magnitude lower than crystalline Si, and these low values limit the use of a-Si:H particularly in thin film transistors. The hole mobilities for these materials, which dictate the solar cell properties, are even lower than the electron mobilities. These lower mobilities have been attributed to the existence of localized "tail states" in the amorphous materials. By crystallizing a-Si:H materials through techniques such as laser annealing, mobilities as high as 100-400 cm.sup.2 Vs have been obtained. However, being able to obtain higher mobilities without having to use post-deposition processing would be desirable and cost efficient.
These a-Si:H based materials also have relatively large numbers of defects as compared with crystalline materials which hinder carrier collection. Over several years of optimization of the deposition conditions of the PECVD process, the defect density levels for the best materials have been decreased to 10.sup.15-10.sup.16 cm.sup.-3. If these levels are further reduced, improvements should be seen in the solar cell as well as other a-Si:H based device properties.
Obviously, improvement in the electronic quality of a-Si:H based materials, in terms of higher carrier mobilities, lower densities of defects and better carrier collection in devices, would advance the use of a-Si:H technology dramatically.
2) Poor Quality Low Bandgap Materials
In order to alter its bandgap, a-Si:H is typically alloyed with various materials as is done of crystalline materials. However, the carrier collection generally degrades when a-Si:H is alloyed with other elements. In respect to the photovoltaics field, a-Si:H is typically alloyed with Ge to lower the bandgap to collect a greater segment of the red part of the solar spectrum. For a-SiGe:H alloys with Ge contents greater than 20%, the photoconductivities and solar cell properties are poorer than those for a-Si:H, with the degradation in properties attributed to a number of factors including the presence of weak germanium-hydrogen bonds, the formation of Ge clusters, the formation of dangling bonds and the emergence of a low density microstructural phase. Improvement in the quality of the amorphous silicon-germanium alloy (a-SiGe:H) material could lead to a significant improvement in the triple-junction solar cells and red light sensors. Several attempts have been made to improve the properties of a-SiGe:H layers and cells through variations in the PECVD deposition conditions. Some progress has been made with higher quality materials prepared at high substrate temperatures, using a substantial amount of hydrogen dilution and under a moderate amount of ion bombardment. However, the optoelectronic and solar cell properties for the a-Si:H alloy are still far superior to those for the a-SiGe:H alloy.
There has been a growing interest in microcrystalline silicon materials as an alternative to a-SiGe:H as low bandgap layers in the multijunction solar cell structure. In particular, a Very High Frequency (VHF) technique has recently been used to prepare novel microcrystalline silicon materials for i-layers in single junction and multi-junction solar cell structures. An important characteristic which makes this microcrystalline material an attractive alternative is that while the material has similar photoconductive properties to the initial properties for the amorphous alloy, these properties do not degrade with prolonged light exposure (&gt;1000 hrs.) as they do for the amorphous material. This degree of stability has been demonstrated in single junction cells whose i-layers were microcrystalline while the n and p-layers were amorphous. These cells had 7.7% efficiencies which did not change after 1000 hrs. of light soaking. Combining this type of cell with a thin a-Si:H blue light absorbing top cell to form a multi-junction cell, efficiencies of 13.1% have been obtained in one attempt to prepare this multi-junction structure. A larger effort to optimize the cell structure involving a number of different laboratories should lead to even higher efficiencies.
While these results are encouraging, there are limitations to the VHF method presently used to prepare the microcrystalline i-layers for the solar cells. The light absorption efficiency for microcrystalline silicon as compared with a-Si:H and a-SiGe:H is low requiring the microcrystalline silicon i-layer thickness to be 10 times thicker than that for the standard a-SiGe:H red light absorbing layers. With the VHF method, the high quality microcrystalline silicon is restricted to a deposition rate around 1 .ANG./s. To use this technique for the production of large area panels using a roll-to-roll machine, the web substrate would have to move 10 times slower through the machine or the machine would have to be roughly 10 times larger than the present day machines. For a batch reactor, the process time would have to be ten times longer. All of these options are economically impractical. Also, the present 13% efficiency is limited in most part by a rather low open circuit voltage (V.sub.oc) due to the low band gap of the microcrystalline silicon. Increasing this bandgap through alloying with another element like carbon should lead to a significant improvement in the device performance. However, preparation of a high quality microcrystalline silicon carbon material has not yet been done using standard PECVD or VHF deposition techniques. Thus, there is a demand for a deposition technique which can be used to produce microcrystalline material with a larger bandgap than the present microcrystalline silicon material at deposition rates of 10 .ANG./s or higher.
As another alternative, thin polycrystalline silicon thin films have been used as the red light absorbing layers in the multi-junction solar cell structure. Stable efficiencies of 11.5% have thus far been obtained. However, use of the polycrystalline material is again hindered by the lack of a high rate deposition technique.
3) Light Degradation of Electronic/Carrier Properties
When high quality a-Si:H materials in devices are subjected to sunlight, the material becomes more defective and the device properties degrade. For example, when the material is incorporated into either an nip or pin solar cell design, this degradation is seen in the form of decreased fill factors, open circuit voltages and cell efficiencies as the cells are subjected to light. Many research groups, including ECD's, have made several efforts to minimize the amount of degradation through alterations in the deposition conditions used in the standard PECVD deposition process. Some advances have been made including the use of hydrogen dilution in the gas plasma to lessen the extent of device degradation. United Solar Systems Corporation of Troy, Mich. has recently shown that for a single-junction a-Si:H nip device, the amount of degradation of the device properties can be reduced by a factor of two through strong hydrogen dilution of the gas plasma during i-layer growth. However even with these improvements, the device properties still degrade by 10-15% from their initial pre-light soaked values.
In the thin film transistor industry, these material instabilities limit the lifetime of the displays to several thousand hours of operation at best. Thus, substantial gains in terms of device efficiencies would be obtained if a-Si:H alloy materials were made to be less susceptible to light soaking.
Attempts to improve the material properties have included a large variation of the deposition conditions used in the PECVD process. However, the gains from recent studies have been rather small and the progress slow moving. In order to solve these deficiencies in the a-Si:H technology, several studies of a-Si:H materials prepared using alternative deposition techniques to the PECVD method have been made. The variety of deposition techniques include electron cyclotron resonance, remote RF plasma and hot wire deposition methods. In most cases the properties for the materials and/or the solar cells have yet to be proven to show properties better than those for PECVD. Besides the material quality, the PECVD technique is easily applied to large scale depositions with relatively uniform depositions obtainable over many square feet. For many of these alternative deposition methods, scaling from the small R&D scale to the large module production level is not straight forward or even possible to meet the demands of uniform large area deposits.
In lieu of an alternative deposition technique for a-Si:H, there is a growing recognition in the PV community that in order to obtain the desired higher stable efficiencies, an alternative thin film PV material must be developed to replace a-Si:H as well as other thin film PV materials (CuInSe.sub.2, CdTe, thin film Poly Si, etc.). This material must have many of the advantageous properties of a-Si:H (low temperature preparation, flexible substrate, simple deposition process, large area deposition capability, etc.) while having the higher stable efficiencies. With these higher efficiencies, a significant reduction in the cost of PV products and technology would be realized which would enable PV to play a major role in improving the quality, cost effectiveness and sustainability of the U.S. energy supply. The improved solar modules will also provide important export opportunities to supply power to the two billion people around the world that lack electricity while maintaining a clean environment. The major barrier to wide scale commercialization of PV to date has been its relatively high cost, and reducing the cost of PV modules is the major challenge confronting the industry. Other, more mature PV technologies, such as crystalline silicon, have reached a plateau, and further substantial price reductions are unlikely. It is generally agreed that a-Si:H or some other comparable thin film technology will be the technology most likely to break significant price barriers and lead to accelerated PV industry growth. U.S. domination of this technology is, therefore, critical to U.S. leadership in a multi-billion dollar market.
Development of this alternative material will also have an impact on several other a-Si:H technologies besides PV which are limited by the properties of a-Si:H, including thin film transistors, linear image arrays and other particle detectors.
To develop these and other materials, a new, fast method of depositing thin film materials is needed. Preferably the new method will deposit the materials at high speed and the deposited materials will be of high quality. The method will also preferably be a PECVD method employing microwaves and controlled ion bombardment of the growing film.